1. Field of the Invention
This invention relates in general to wellhead assemblies, and in particular to a wellhead assembly having a casing hanger located in a wellhead, a seal member which seals a space between the casing hanger in the wellhead, and test passages extending through the seal member and wellhead for testing the seal.
2. Description of the Prior Art
One type of wellhead assembly employs a casing hanger which secures to the upper end of a string of casing that is cemented in the well. A seal or packoff seals the annulus between the casing hanger and the Wellhead housing.
During installation, the seal member can be tested by using a test tool which seals in the bore of the suspended casing. This allows pressure below the blowout preventer to test the seal from above. After the well has been completed, it is desirable to periodically check to determine if the casing seal has started to leak. There is no access to the bore of the wellhead housing above the casing hanger after completion.
To handle this, some installations have an external test port extending through the wellhead housing. A special test seal will be located below the primary seal. The test port communicates with the space between the test seal and the primary seal. Pressure is applied. If the pressure holds, then that indicates that neither seal is leaking.
One drawback to the prior art procedure is that the test seal may leak, but not the primary seal. The operator would not know which seal leaks. The test seals are normally not as effective or long lasting as the primary seal, and thus are more likely to leak.
This disadvantage is particularly a problem in the case of metal-to-metal seals. Metal-to-metal seals do not deteriorate as much with age as elastomeric seals. One type of metal seal available has inner and outer cylindrical walls separated by an annular cavity. During the installation, an energizing member moves down into the cavity to force the walls apart into tight sealing engagement with the wellhead housing bore and the exterior of the casing hanger. No means exist for testing these types of metal seals, other than providing some type of elastomeric seal below the metal seal.